Flourless French Chocolate Cake

Flourless French Chocolate Cake
4 squares (one ounce or 28 grams each) bittersweet chocolate
1 cup semisweet chocolate chips
3/4 cup butter
5 Tablespoons granulated (white) sugar
2 teaspoons vanilla extract
5 large eggs, separated
dash salt
3 Tablespoons granulated (white) sugar
1/4 cup all-purpose (plain) flour 
(optional, but provides a little more stability, for a truly flourless or gluten free chocolate cake, omit or substitute with a gluten-free flour or ground nuts)

Preheat oven to 325◦F. Spray a 9-inch springform pan with cooking spray, line with parchment paper, and spray paper. 
In a medium saucepan over medium heat, melt the chocolate, butter, and 5 Tablespoons sugar until smooth. Stir in the vanilla, and set aside to cool slightly.
Meanwhile, beat the egg whites until foamy. Add the salt and beat until soft peaks form. Add 3 Tablespoons sugar and beat until stiff peaks form.
Beat the egg yolks, one at a time, into the chocolate mixture. Beat in the flour.
Beat 1/3 of the egg whites into the chocolate mixture. Fold in the remaining egg whites. 
Spread batter evenly in prepared pan. Bake for about 35-45 minutes, until a toothpick inserted in the center comes out clean. Cool completely before dislodging from the pan.